Automatic speed control devices are highly advantageous in driving great distances on limited access highways to help the driver maintain a desired speed for a long period of time.
The earliest automatic speed control was an elementary dashboard throttle control positioned to hold the throttle at a fixed setting. This type of control, however, was unable to compensate for the various changes in driving conditions, such as wind, terrain, and road surface characteristics.
Electronic automobile speed controls have been developed using two analog electrical signals, one proportional to the actual speed of the vehicle and the other proportional to the desired vehicle speed. These analog signals are compared to generate an error signal which is applied to position the throttle to make the actual speed of the vehicle equal to the desired speed.
Further improvements to these prior art devices have been disclosed which utilize analog memory systems for recording the deisred vehicle speed. A typical analog memory comprises a high quality capacitor which is charged to a preselected voltage. The voltage stored by the capacitor is proportional to the desired speed. A very high input impedance amplifier is used to read out the preselected voltage and compare it to the voltage representative of the actual speed of the vehicle. The high quality capacitors must be able to maintain their charge within one percent for five hours. To accomplish this, the capacitors must be of an expensive variety and be hermetically sealed to prevent leakage. The high cost of these capacitors is therefore highly disadvantageous.
Another example of an analog memory is disclosed in U.S. Pat. No. 3,340,950 by Albert Hopengarten. Hopengarten discloses an analog memory for speed control wherein a tone having a frequency proportional to the actual speed of the vehicle is generated. When the desired speed is reached, the tone produced at that speed is recorded on a magnetic tape. Means are provided for controlling the speed of the vehicle in response to the recorded signal.
Analog memories such as those described above are expensive in comparison to a digital memory. However, using the digital memory in an analog system would require analog--to--digital converters which would destroy the inherent economy of a digital memory.
It would be desirable, therefore, to construct an all digital speed maintaining system to take full advantage of the economy of a digital memory. In addition, the emerging technology of metal oxide semiconductor large scale integration (MOS LSI) circuits makes the digital approach economically attractive for programs involving large quantities, such as automobile production. Furthermore, the nature of digital electronics results in a speed control that is largely unaffected by temperature variations, power supply noise, aging and production tolerances.